The invention relates to polymerizable diazonium salts which have redox properties and properties of absorption in the visible range at various wavelengths, to the process for preparing them and to uses thereof.
The functionalization, also referred to in the subsequent text as modification of electrically conductive or semiconductive surfaces with polymer films finds application in many fields, such as biology, electronics and optical components.
At the present time, there is an interest for at least two types of conductive or semiconductive surface modifications:                firstly, the modification of conductive or semiconductive surfaces with metal complexes, and        secondly, the functionalization of conductive or semiconductive surfaces using diazonium salts.        
The modification of conductive or semiconductive surfaces with metal complexes results in hybrid devices and is of great value in the field of energy conversion, information storage, optics, or molecular electronics.
In fact, the properties and the behaviour of these hybrid electronic or optoelectronic devices are greatly influenced by the electron exchanges between the organic and inorganic part, and, consequently, by the nature of the chemical bonds between the organic molecules and the semiconductor and also by the amount and the nature of the grafted metal complex.
Thus, in the field of energy conversion, and more particularly in Grätzel photovoltaic cells, the semiconductive surface is modified with metal complexes.
In this case, the photoconversion is carried out by means of metal complexes which are bound to nanoparticles of TiO2, SnO2, ZnO or ZrO2.
In these Grätzel cells, the ligands of the complexes comprise phosphonates, siloxanes, ethyl malonate, ether and/or cyanide functions which interact chemically with the surface of nanoparticles of TiO2, SnO2, ZnO or ZrO2 which become grafted by means of bonds that are covalent or ionic in nature and give monolayers of complexes at the surface.
The metal complex in this type of device is the active component since it absorbs visible light and performs the charge separation.
In fact, photoexcitation of the metal complex (photosensitizer) which absorbs light produces the injection of one of its electrons into the conduction band of the semiconductor. The reduction of the oxidized complex leads to the formation of positive charges which are transported to the counter electrode by means of the conductive electrolyte (I3−/I−). The oxidized photosensitizer is then reduced by the I3−/I− couple of the electrolyte so as to return to its initial state.
The energy which will be generated by this photovoltaic cell will therefore depend on the number of photons absorbed by the metal complex grafted at the surface of the nanoparticle.
However, the technique for grafting complexes in this type of device only makes it possible to produce monolayers of complex and therefore does not make it possible to absorb the entire intensity of the solar spectrum.
Furthermore, the use of the I3−/I− electrolyte, which is a liquid, poses a major problem. This is because it is difficult to obtain completely hermetic cells in which there is no leaking of the liquid electrolyte.
Moreover, in the context of information storage, the semiconductive surface of silicon is modified by the grafting of redox molecules, such as porphyrins or ferrocenes, onto this surface.
The organic compounds grafted onto these surfaces are the active compounds of the device since they are those which will be responsible for the memory effect through charge storage.
However, the methods of functionalization developed up until now are exclusively methods which involve chemical reactions such as the thermal activation of an alcohol, of a thiol, of a methyl halogenated derivative or of thiols that are protected with a surface of hydrogenated silicon or of halogenated silicon, the thermal hydrosyllilation of metal complexes comprising an alkene or alkyne function.
All these methods have various drawbacks, in particular the fact that the surface functionalization with these compounds is carried out at high temperatures, of the order of 200° C. to 400° C., conditions which are not compatible with all chemical compounds.
Furthermore, these reactions can also be extremely expensive in terms of product when they are carried out under solid conditions.
Similarly, with these various methods, the metal centre which has the redox properties is set apart by an arylmethylene bridge which distances it from the surface.
Finally, all these methods result only in the formation of monolayers and not polymers.
However, WO 2005/86826 A2 describes the formation of redox polymers on a silicon surface. The chemical method used consists in heating porphyrins comprising two alkyne functions to a temperature of between 200° C. and 400° C.
While said document indeed describes the formation of redox polymers on a conductive surface, this formation is again carried out by means of a reaction at high temperatures and cannot therefore be applied to all organic compounds.
Another means of functionalizing conductive or semiconductive surfaces is the electrochemical functionalization of these surfaces with diazonium salts.
Electrochemical functionalization with diazonium salts makes it possible to obtain polymers grafted to the surface, generally of silicon. It has been carried out using commercial diazonium salts which are 4-nitrophenyldiazonium tetrafluoroborate or 4-bromophenyldiazonium tetrafluoroborate.
The specific property of these diazonium salts is that they are grafted covalently with silicon and therefore form a strong interaction between molecules and substrate.
However, this method is limited since few diazonium salts exist because they are compounds that are unstable at ambient temperature and difficult to isolate.
Furthermore, the polymers formed and also the semiconductive surfaces formed cannot be used for information storage or energy conversion.
In fact, at the present time, no diazonium salts exist which could absorb visible light at various wavelengths and have redox or photoredox properties.
Another problem of this method, still due to the specificity of the diazonium salts used, is that the films deposited at the electrode by this method are thin. This is due to the deposition of nonconductive compounds on the surface, which do not make it possible to provide the charges in order to continue the electrochemical deposition, and therefore result in the deposition of an insulating film at the surface of the semiconductor.
This method is known in the literature for the functionalization of silicon surfaces, i.e. narrow-band semiconductive surfaces.
However, this method has never been described for the modification of wide-band semiconductors such as TiO2, ZnO, SnO and ZrO2.